Arsenic(III) sulphide, As2S3: The arsenic must be present as arsenic(III). In this condition [ensured by the addition of reductors such as iron(II) sulphate, copper(I) chloride, pyrogallol, or phosphorous(III) acid] arsenic may be separated from other elements by distillation from a hydrochloric acid solution, the temperature of the vapour being held below 108 °C; arsenic trichloride (also germanium chloride, if present) volatilises and is collected in water or in hydrochloric acid.
Pass a rapid stream of washed hydrogen sulphide through a solution of the arsenic(III) in 9M hydrochloric acid at 15-20 °C. Allow to stand for an hour or two, and filter through a weighed filtering crucible (sintered glass or porcelain). Sometimes a film of the sulphide adheres to the glass vessel in which precipitation was carried out; this can be dissolved in a little ammonia solution and the sulphide re-precipitated with the acid washing liquor. Wash the precipitate with 8M hydrochloric acid saturated with hydrogen sulphide, then successively with ethanol, carbon disulphide (to remove any free sulphur which may be present), and ethanol. Dry at 105 °C to constant weight, and weigh as As2S3.